1. Field of the Invention
The present invention relates generally to a vehicular hydraulic brake apparatus and, more particularly, to a vehicular hydraulic brake apparatus of the type that has a tandem master cylinder having front and rear sections connected through independent front and rear wheel braking hydraulic circuits to front and rear wheel brake cylinders, respectively, and a deceleration-responsive braking pressure control device.
2. Background of the Invention
A vehicular hydraulic brake apparatus of the type referred to is disclosed, for example, in U.S. Pat. No. 3,771,835. The brake apparatus comprises a tandem master cylinder, a front wheel braking hydraulic circuit connecting a front section of the master cylinder to front wheel brake cylinders, a rear wheel braking hydraulic circuit connecting a rear section of the master cylinder to rear wheel brake cylinders, and a deceleration-responsive braking pressure control device provided in the rear wheel braking hydraulic circuit. The braking pressure control device has pressure inlet and outlet between which is provided a deceleration-responsive valve operative to interrupt the communication between the inlet and outlet when the deceleration of an associated vehicle reaches a predetermined level. The pressure control device also has a differential piston having a first pressure-receiving area upon which acts the hydraulic pressure in the pressure inlet and a second pressure-receiving area which is larger than said first pressure-receiving area and upon which acts the hydraulic pressure in the pressure outlet.
In the case where both front and rear hydraulic circuits operate without fail, the deceleration-responsive valve is closed when a brake pedal is actuated until the vehicle is decelerated to a predetermined deceleration. Thereafter, if the brake pedal is continuously actuated, the hydraulic pressure in the pressure inlet of the hydraulic pressure control device (input pressure) continuously increases and acts on the first pressure-receiving area. As a result, the differential piston is forced toward the pressure outlet. Since the second pressure-receiving area of the differential piston is larger than the first pressure-receiving area, the hydraulic pressure at the pressure outlet of the hydraulic pressure control device (output pressure) increases at a rate which is dependent upon the ratio of the first pressure-receiving area to the second pressure-receiving area. The rate of the pressure increase is thus smaller than the rate of the output pressure increase obtained prior to the closing of the deceleration-responsive valve. It is well known in the art that this output pressure increase characteristic is suitable for rear wheel brakes.
In case where the brake liquid leaks from the front wheel braking hydraulic circuit due to damage thereof, the deceleration of the vehicle must rely solely on the rear wheel brakes (this condition is called "rear partial".). As a result, a large quantity of brake fluid must be forced into the rear wheel brake cylinders. In this case, since the deceleration-responsive valve is not closed before the deceleration of the vehicle reaches the predetermined value, the flow of brake fluid into the rear wheel brake cylinders will not be interrupted until the deceleration reaches the predetermined value. When the hydraulic pressure in the rear wheel braking hydraulic circuit rises beyond a predetermined magnitude, the force acting on the second pressure-receiving area of the differential piston exceeds the force acting on the first pressure-receiving area so that the differential piston is caused to move in the upstream direction, i.e., toward the pressure inlet of the pressure control device. This means that a part of one complete stroke of the brake pedal is consumed for the displacement of the differential piston toward the pressure inlet of the pressure control device, but not for driving the pistons in the rear wheel brake cylinders. In other words, a part of the flow of the brake fluid produced in the rear section of the tandem master cylinder is wasted without any braking force. In such an event, there is an insufficient margin left to the pedal stroke with a resultant risk that the hydraulic pressure in the rear brake cylinders is not increased sufficiently to obtain a required vehicle deceleration.